We lift our foot from the accelerator and the engine stops running. A likely cause is a failure of the engine to idle. How do we adjust the engine idle speed to correct the problem? The answer is a bit more complex than most folks realize.

How do we adjust an engine's idle speed?

On carbureted vehicles, we increase and decrease the engine idle speed by simply turning an adjustment. Vehicles built today have no such provision. An engine’s idle speed is a programmed function. We cannot directly adjust the engine idle speed on a modern vehicle. The power control module or PCM provides all adjustments to the idle speed.

Though no external engine idle speed adjustments are possible, many things can and do cause problems. Should the speed of the engine idle need adjustment, we do it by diagnosing and correcting the things that cause it to be incorrect. After we correct the problems, the PCM will adjust the engine idle speed to the correct setting.

What is the engine idle?

Revolutions per minute, or RPM, is the most common measurement of engine speed. The idle speed is the RPM of an engine, before we open the throttle. This is also the lowest speed at which the engine operates. When a driver releases the accelerator, the engine returns to the idle speed. If the engine fails to idle, it will die.

Idling is an inefficient but necessary operating speed. Seeing a slight movement on the tachometer is normal, as the engine idles. At idle speed, the engine lacks the inertia to smooth out the lag between the operating cycles. If the idle speed is too low, the engine will stall. Setting the idle speed higher than needed will waste fuel and may cause engine run-on when we turn the ignition off. Design engineers closely regulate the idle speed, depending on several factors.

What is the proper engine idle speed?

Vehicle manufactures specify an idle speed for each vehicle they build. The specification they give, is for an engine that is fully warmed up and operating normally. We must check the specifications for the vehicle to know the proper idle speed. An engine that feels rough, at the proper idle speed, usually has a problem other than the idle system. Normal engine vibration transfers to the passenger compartment with bad engine mounts. This may feel like the engine roughly idles, but bad engine mounts are the cause. The idle system only controls the speed of the idle and not how smoothly the engine runs.

The actual speed of the engine idle will vary, depending on the temperature of the engine. An engine that is not warmed up requires a higher idle speed. When at normal operating temperature, they reduce the idle speed. For instance, an engine may normally idle at 650 RPM but will go to 1,000 RPM or more when cold. Cold engines run with less efficiency and require a higher idle speed. The higher RPM also allows better lubrication and a faster warming of the engine.

A brief history of idling an engine

Years ago, they used carburetors on most vehicles. With a carburetor, we can adjust the idle speed and usually the idle fuel mixture. Multiple adjustments are possible and routinely needed on most carburetors. The carburetor dispenses fuel, based on air flow through the venturi. More air flowing through the venturi will draw more fuel into the engine. They control the ratio of air to fuel using a jet that allows only a certain amount of fuel to pass through.

At an idle, air flow through the carburetor is not sufficient to draw the fuel in, like it does when it is running. Small passages, below the throttle plates, allow the engine vacuum to draw the fuel in. We can usually adjust the air/fuel ratio by turning the idle mixture adjustment-screw. Turning the screw in reduces the fuel and turning it out adds more fuel. Most designs also add a small slot, above the throttle plate. Additional fuel flows through this slot to smooth the transition from idle to running speed.

Carburetors use another screw that pushes the throttle open to control the idle speed. As the throttle opens, air flows in and engine speed increases.

With the carburetor, engineers increase cold-engine idle speed with an ingenious cam setup. On most engines, a thermostatically controlled flap, called a choke, restricts airflow when the engine is cold. A cam attaches to the choke shaft and a separate idle speed adjustment contacts this cam. When the engine is cold, the choke rotates the cam, and engine RPM increases. This works well, but is prone to problems and requires periodic adjustment.

Engine idle speed on injected engines

With electronic fuel injection, we do not rely on air flow to draw fuel into the engine. Sensors provide inputs to the power control module or PCM, based on temperature, air flow, throttle position and much more. The PCM controls the idle speed, by complex computations and sensor inputs. Fuel is added, by opening the injectors and the amount matches the air flow. This is far more accurate than the carburetor and is not adjustable.

Cold engine idle speed and the PCM

Various sensors provide data to the power control module. They base calculations on barometric pressure, throttle position, engine and ambient temperature and more. Based on these and other inputs, the PCM decides which idle speed is best, according to its program. The designer determines engine idle speed and changes are not possible without modification of the software.

When the engine is warm, the air/fuel sensors provide feedback. They tell the PCM how much oxygen remains in the exhaust. Too much oxygen means incomplete combustion and the PCM makes adjustments, by reducing the fuel. If the oxygen in the exhaust is too low, additional fuel is added.

Air/fuel and oxygen sensors do not operate below a certain temperature. On a cold engine, this input is not present and the PCM uses default programming and available data. This is why a vacuum leak causes the engine to roughly idle when cold. When the engine reaches operating temperature, the air/fuel sensor detects the lean condition and the PCM adds more fuel. If PCM requests too much additional fuel, the check engine light comes on, noting a problem.

How an engine changes the idle speed

The method used by the PCM to raise and lower the idle speed of the engine depends on vehicle design. We cannot adjust or change the idle speed. The speed of engine idling is only adjustable by the PCM. On older vehicles an idle air control valve or servo is the most common method. The PCM commands the device to open, when the engine idle speed is too low. This adds air to the engine and RPM increase.

Most modern vehicles use automated throttles or drive-by-wire. On this system, the throttle body is an output from the PCM, rather than an input to it. To increase the idle speed the PCM commands the throttle-body actuator to open and RPM increase.

Why is the idle speed incorrect?

We cannot manually adjust the engine idle speed, but several things can and do go wrong with the idle speed control system. The engine designers program idle speed into the PCM, but it is also learned based on driving. A memory recalls driving conditions and provides final idle speed adjustments. When we disconnect the battery or when the battery dies, we lose the idle memory.

The power control module will begin collecting data immediately, but it takes time to regenerate the adjustments. While relearning the engine idle speed, the engine may stall and run erratically. Once learning is complete, the PCM will control the idle speed and it needs no driver input.

A dirty throttle body can keep the PCM from relearning engine idle speed. Carbon builds up on the back of the throttle body blade. This may keep the blade from fully closing. The PCM learns and adapts to the incorrect position as we drive the vehicle. When memory is cleared, the engine is unable to reestablish engine idle memory, due to the corrupt input. Such an engine may not return to the proper engine idle speed until we correct the problem. Cleaning the throttle body and relearning the idle speed will normally correct the problem.

A bad sensor can have the same result. The engine idle speed memory may allow the engine to run without the sensor’s input, but the PCM cannot adjust idle speed with the missing data. The driver of the vehicle may be unaware of this problem, until the battery power is lost. Now the engine begins to die, because the engine idle memory is now gone. A bad sensor caused the problem, but engine idle memory covered it, until we disconnect the battery. Once engine idle memory is gone, the PCM can no longer compensate for the problem.

A generic procedure to reset the idle speed

Even though we cannot manually adjust engine idle speed on a modern engine, it is possible to reset the PCM for improved engine idle. Every vehicle has a unique relearn procedure and fortunately, most will need no input from the driver. This generic procedure will often help an engine that fails to relearn the idle speed.

After we replace the battery and resolve other problems, we turn off all accessories such as air conditioning. We start the engine and allow it to run for two minutes in neutral. When the idle speed stabilizes, put the vehicle in drive, with the brakes applied. Again allow it to run for about two minutes or until the idle speed stabilizes.

We again return the vehicle to neutral and turn on the air conditioner. Engine speed may drop, but should stabilize and smooth out after about two minutes. With a foot on the brake, shift into drive and allow the engine to run at idle speed, with the air conditioner on. This should help the PCM to learn base idle speed.

Other engine idle speed problems

Most other idle speed issues will set a check engine light. A few possible causes of incorrect idle speed include, a bad or missing sensor input, a sticking idle control valve or a bad PCM. We need diagnosis to learn the causes of these problems. Diagnosis involves testing the inputs to the PCM and the outputs from it. This diagnosis requires training and test equipment. Having a professional diagnose the problem is normally far less expensive than replacing parts.